google-map-marker: update if condition in _dragEventsChanged method to check for marker instead of map

[paodb]

While working on GoogleMaps issue #40 I run into a JS error "Uncaught TypeError: Cannot read property '_e3' of undefined" issue when trying to add a listener for google-map-marker-dragend event. After some debugging I found that the issue is in google-map-marker.js file, in method _dragEventsChanged(). The if condition should be if(this.marker) instead of if(this.map). If marker is undefined at this point there should be no call to this._forwardEvent('dragend'); that is what is making it fail.